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I have a '67 T337B. It holds 118 gals. total, with 92 in the mains and the rest in the aux. tanks.
I fly between 70% and 75% almost all of the time, which is between 23-25 GPH.
When I'm not making a long flight my usual practice is to leave 7 gals. in each aux. tank. That means if I do something really dumb I can throttle back and still get a half hour of flying. (Rarely would I ever be in a situation where I couldn't find some place to set down in half an hour.)
If I top all tanks off I should theoretically be able to get just under 5 hours before something stops spinning. Of course I never go anywhere near that. After 3 - 3 1/2 hours I usually am ready for a stop anyway.
I always fill up when the plane is level, check the "top off" with a ladder, and if I've filled up the aux. tanks for a longer flight, I'll run down [when an engine starts missing I switch back to the main - warning any passenger(s) just in case they might otherwise be nervous - and woth my model's fuel system it's perfectly safe to do this] so that when I land I can have them measure 7 gals into each aux. again with their fuel truck meter set to zero.
After I flew the plane a few times with all the tanks full, it didn't take long to see how much fuel it took when I topped them off again. Then I could make fairly accurate estimates of what kind of flying time I have. If a 3 hour flight (since I always use pretty much the same settings) means I consistently fill up with 75 gals. and the aux tanks had 14 total in them when I started, then I know I had 31 gals. left. That's over an hour, and for me that's a good comfort level.
I just make it a practice not to start a trip, generally even a very short one, without having the mains topped off, so that I have a reasonable guide as to the endurance available.
If you've done this time after time and you otherwise watch the settings (and if your fuel gages are good enough to give you at least a ballpark backup indication), then relying on the time you fly can provide a reasonable margin - even without the more expensive fuel computers/gages.
I did almost screw up once - I was doing a lot of practicing and short flights and didn't refill when I should have: the rear engine quit just on the landing rollout and it was a real surprise. I could have switched to the aux. tank earlier and then there would have been fuel in the main for the landing, but I don't like ever getting that close anyway. If you do use the aux tanks (I guess some models don't have aux. tanks?) then you need to use them in the earlier portion of a flight so that your mains are available for landing, of course.
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